Microwave circuits are generally comprised of a substrate of dielectric material having a conduction layer extending over all of one side that serves as a ground plane and circuit devices and interconnections therefor on the other. With superconducting circuits operating at millimeter wave lengths, the high dielectric constant of the substrate leads to small circuits having high current densities so that device coupling and high losses may result. In such circumstances it has been found advantageous to mount the circuit on one side of a dielectric substrate and to form the ground plane on one surface of another substrate that is mounted in spaced parallel relation to the first in such manner that the circuit and ground plane face each other. Thus, air which has the lowest possible dielectric constant and lowest loss tangent lies between the ground plane and the circuit so as to increase circuit size and decrease losses. This is called an inverted microstrip structure.
One of the problems encountered with inverted microstrip circuits is the fact that circuits may respectively require different separations between them and their ground planes for optimum performance; this is done to .change the characteristic impedance of the medium through which the wave travels. It would therefore be highly advantageous to know the optimum separation before the circuit assembly is finally designed. At the present time, no known means is available for determining the optimum separation in advance.